A variety of non-isolated and transformer-isolated buck-boost switching power converters are known in the art. In general, a buck-boost switching power converter can generate a pre-determined output voltage as its input voltage varies over a range that includes values both less than and greater than its output voltage (in the case of transformer-isolated buck-boost converters, the ratio of input to output voltage is normalized to the transformer turns ratio). In contrast, the maximum or minimum converter output voltage may be limited by the value of input voltage in other converter topologies (e.g., in a canonical buck converter the normalized output voltage cannot be greater than the input voltage; in a canonical boost converter the normalized output voltage cannot be less than the input voltage).
Flyback converters and isolated Cuk converters are known examples of isolated buck-boost switching power converters. Flyback converters using active clamp circuitry are known (see, e.g., Jitaru, “High efficiency flyback converter using synchronous rectification,” APEC 2002, Volume 2, pp. 867-871; Dalal, “Design Considerations for Active Clamp and Reset Technique,” 1996 Unitrode Design Seminars, SEM1100).
Using a switch to retain energy in an inductive element as a means of reducing noise and switching losses in switching power converters is described in Prager et al, “Loss and Noise Reduction in Power Converters,” U.S. Pat. No. 6,522,108, issued Feb. 18, 2003, assigned to the same assignee as this application and incorporated in its entirety by reference. A buck-boost converter incorporating switches to retain energy in an inductive element and control techniques for operating the converter is described in Vinciarelli, “Buck-boost DC-DC switching power conversion,” U.S. Pat. No. 6,788,033, issued Sep. 7, 2004, assigned to the same assignee as this application and incorporated in its entirety by reference.